OK - this is solved. The problem turned out to be that I am an idiot. A number of the guides online caution that if the zwave device uses tty and the openhab user does not have root privileges then they need to be added to the “dialout” group. I didn’t create an openhab user and my default user is a member of dialout - so I thought that was that. But then looking at the running processes I see that apt-get did create an openhab user, so I do need to add to the dialout group.
Everything is now behaving.
Many thanks for all your help, and apologies for wasting your time.